1. Field of the Invention
The present invention relates to analog/digital converters, and also relates to illuminance sensors, illumination devices, and electronic devices employing them. More particularly, the present invention concerns with, in such devices, widening the input dynamic range, enhancing the minimum resolution, and shortening the measurement time.
2. Description of Related Art
As a technology related to analog/digital converters, JP-2001-160756 discloses and proposes an analog/digital converter as shown in FIG. 7, comprising a capacitor for storing an electric charge commensurate with a measurement-target input voltage level, a constant current circuit for discharging the stored electric charge, a counter for counting clock pulses after the start of discharging until the voltage across the capacitor becomes equal to a predetermined level.
Certainly, with the conventional analog/digital converter mentioned above, it is possible to achieve, with a simple configuration, analog/digital conversion processing of a measurement-target input voltage level.
Inconveniently, however, with the conventional analog/digital converter mentioned above, the higher the measurement-target input voltage, the longer it takes to discharge the capacitor, and thus the longer the conversion time required.
Increasing the current level through the constant current circuit may shorten the time needed to discharge the capacitor, but doing so necessitates, to maintain the minimum resolution of conversion, increasing the frequency of the clock.
On the other hand, in the field of illuminance sensors, there is conventionally adopted a configuration as shown in FIG. 8, comprising photodiodes PDa and PDb with different photodetective characteristics (spectral sensitivity), wherein photoelectric conversion processing is performed on a time-division basis and, after the output current from each of the photodiodes PDa and PDb is processed by analog/digital conversion processing, the difference between the two currents is calculated to measure the targeted wavelength alone.
One disadvantage with the conventional configuration mentioned above is that a calculation circuit needs to be provided separately, leading to an increased circuit scale.
Another disadvantage with the conventional configuration mentioned above is that the output currents from the photodiodes PDa and PDb are fed directly to an integrating operational amplifier without their difference being calculated beforehand, resulting in a narrow input dynamic range.
Still other disadvantages with the conventional configuration mentioned above are that, since photoelectric conversion with the photodiodes PDa and PDb is performed on a time-division basis, measurement takes twice as much time as with a single photodiode, and that a variation in illuminance between during measurement with the photodiode PDa and during measurement with the photodiode PDb produces an inaccurate calculation result.
These disadvantages may be overcome by completely separately providing an analog/digital converter for the photodiode PDa and an analog/digital converter for the photodiode PDb, but doing so doubles the circuit scale, quite disadvantageously for application in, for example, portable devices.